Mobile data transmission and data services are constantly making progress. With the increasing penetration of such services, data throughput and transmission reliability find more and more attention.
Under one aspect, investigation is made in scenarios for mobile communication which comprise a plurality of transmission points, each constituted by a set of at least one transmit antenna, for transmitting data to another device in a coordinated transmission from at least a subset of the plurality of transmission points.
It should be noted that concepts outlined in connection with the present invention are generally independent of any particular communication standard; rather, they are generally applicable to a variety of compatible standards. In order to properly describe the concept(s), however, for explanatory purposes only and without any intention to limit the applicability of the concept(s) introduced in the specification to a particular standard, those concept(s) are described with reference to an example scenario. As the example scenario, LTE (Long Term Evolution) and/or LTE-A (LTE-Advanced) was chosen for the network infrastructure.
That is, e.g. in the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) and LTE-Advanced (LTE-A), single cell single-user (SU-) and multi-user (MU-) multiple-input multiple-output (MIMO) network performance is interference-limited, especially at the cell edge.
Therefore, introduction of the coordinated multipoint (CoMP) transmission/reception technology has been considered, where in downlink (from a network device such as an eNB (evolved NodeB) towards a terminal such as a user equipment UE), multiple points cooperate in scheduling and transmission in order to strengthen the desired signal and to mitigate inter-cell interference.
According to e.g. the 3GPP technical report (TR) on CoMP, TR36.819, a point is defined as a set of geographically co-located transmit antennas and the sectors of the same site correspond to different points. It should be noted that a cell is formed by one or multiple points.
The above mentioned CoMP TR was approved after a recent RAN meeting. The agreed CoMP working item definition proposes the following focus for the CoMP work during a subsequent release (e.g. Rel-11):
“The work for specifying CoMP support in Rel-11 should focus on                Joint transmission (JT)        Dynamic point selection (DPS), including dynamic point blanking        Coordinated scheduling/beamforming (CS/CB), including dynamic point blanking”.        
In joint transmission (JT) CoMP, two or more points transmit simultaneously to a CoMP user. Dynamic point selection (DPS) on the other hand refers to a scheme where the transmission point is switched according to changes in signal strength. In coordinated beamforming/scheduling (CB/CS), in turn, the scheduling decisions of neighbor points are coordinated in order to reduce interference. In principle, all schemes may include blanking/muting which means that one or more transmission points are blanked/muted (switched off or not used for transmission) to decrease the interference.
The agreed CoMP working item targets specification of intra-cell and inter-cell DL CoMP schemes which operate in homogeneous and heterogeneous configurations. Four main scenarios have been studied so far: intra-site (scenario 1), inter-site with high power remote radio head (RRH) (scenario 2), low power RRH within the coverage of the macro cell, without and with the same cell ID (scenarios 3 and 4, respectively).
CoMP working item addresses both frequency division duplex FDD and time division duplex TDD. Hence, unified solutions should be targeted, as it is always the case in LTE specifications.
CoMP is intended to improve the performance of cell edge users, as especially at cell edge the performance is interference limited. A CoMP measurement set is formed by M cells/points for which the UE is measuring channel state information. The reporting set has been limited to N cells/points defining the number of points for which CSI feedback is reported. A common assumption has been that the CoMP reporting set is formed by two to three points. Also the CoMP reporting set could be equivalent to the CoMP measurement set. The number of points involved in CoMP scheme (cooperation set) does not need to be signaled to the UE or mentioned in specifications but is left for network implementation. The point from which the UE would receive transmission in single-cell mode is defined as the serving point.
In Release 10, different reference signals (RS) were defined for CSI estimation and data demodulation purposes. Namely, channel state information reference symbols (CSI-RS) and demodulation reference symbols (DM-RS).
Such reference symbols are assigned to (specific) physical resource elements RE within physical resource blocks PRB. A resource element RE is represented by a time slot and a frequency (bandwidth) assigned to it within the frequency-time domain. A plurality (defined number) of resource elements in frequency/bandwidth domain form a physical resource block PRB (in frequency domain), and a plurality of PRBs are present within a channel.
PDSCH (Physical downlink shared channel) resource element muting is also specified, allowing for multi-cell channel estimation. The baseline feedback has been agreed to be implicit feedback which consists of rank indicator (RI), precoding matrix index (PMI) and a channel quality indicator (CQI). Hence, the UE estimates the channel, selects rank and PMI and calculates the post-processing (after receiver) SINR (signal to interference noise ratio) and derives the CQI based on that. CQI may be seen as indicative of the post processing SINR. Release 10 feedback operates per point. The CoMP specific flavors are that a UE may receive CSI-RS resources from more than one point and it is possible to design aggregated (over multiple CSI-RS resources) or per point (per CSI-RS resource) feedback. The per-point PMIs may be improved by a combiner feedback that may be an inter-point phase and/or amplitude value.
Table 1 summarizes the feedback and channel estimation options for each CoMP scheme.
TABLE 1Feedback for different CoMP schemesCS/CB +JTDPS + mutingmutingFeedback1) Per pointPer pointPer pointPMI/CQI (+PMI/CQI +PMI/CQI orcombiner)point selection +CS/CB +2) Per point PMIpossiblemuting specific(+combiner) +mutingadditionalaggregated CQI +indicationfeedbackserving pointCQI3) AggregatedPMI/CQI +serving pointPMI/CQIChannelPer point CSI-RSPer point CSI-Serving pointestimationor aggregatedRSCSI-RS +single CSI-RSpossibly otherpatternpoint CSI-RS
During a recent RAN meeting, the following working assumption was agreed:
“Definition: “CSI-RS resource” here refers to a combination of “resourceConfig” and “subframeConfig” which are configured by higher layers.
Standardize a common feedback/signaling framework suitable for scenarios 1-4 that can support CoMP JT, DPS and CS/CB. Feedback scheme to be composed from one or more of the following, including at least one of the first 3 sub-bullets:                feedback aggregated across multiple CSI-RS resources        per-CSI-RS-resource feedback with inter-CSI-RS-resource feedback        per-CSI-RS-resource feedback        per cell Rel-8 CRS-based feedback        
Note that use of SRS sounding reference signal (SRS) used in uplink measurement may be taken into account when reaching further agreements on the above.”
The CoMP problem relates mostly to the CQI feedback. The CQI is used by the eNB to perform adaptive modulation and coding which means the transmission rate is adapted based on channel conditions. Accuracy of the CQI value affects greatly on the system performance, especially if the CQI is overestimated and too high transmission rate is assigned which is not supported by the actual radio link. The CQI depends on the transmission hypotheses made by UE at a given time. For example:                When reporting an aggregated JT CQI, UE assumes combined transmission from N points to the UE,        When reporting a DPS CQI without muting, UE assumes transmission from a selected transmission point and interference from other transmission points,        When reporting a DPS CQI with muting, UE assumes transmission from one point and zero interference from points that are assumed to be muted,        When reporting CQI for CS/CB, UE assumes transmission from one point and reduced interference or muting from other transmission points.        
In addition to the transmission hypothesis, the CQI value depends on the hypothesis of the interference.
Generally, current activities in CoMP related research target to designing common feedback for the CoMP schemes.
In current TR 36.819, which was agreed after a recent RAN1 meeting, a baseline feedback is individual per-point feedback with or without complementary inter-point feedback. Several contributions indicate the need of unified feedback, though several contributions did not present any possible solutions but only stated a viewpoint in terms of a need of a unified solution.
Further, a hierarchical feedback structure to support CS/CB, DPS and JT CoMP was presented. In this regard, proposed is a fallback feedback for serving cell and additionally RI/PMI/CQI to other points, and if JT is configured, also inter-point phase and/or amplitude. The PMI reports to other points support CS/CB as such, and the per point CQIs are assumed to support also the possible JT transmission using eNB implementation-based CQI adaptation mechanisms. However, thus no aggregated CQI for JT is discussed. Muting assumptions for the CQIs were not discussed and additional feedback to support dynamic muting (blanking) was left for further study (FFS).
Another contribution proposes, for JT, a per CSI-RS resource feedback and aggregated CQI corresponding to JT hypothesis. For DPS, per point (CSI-RS resource) CQI is proposed based on which network decides the transmission point. Additionally, CQIs for different muting options may or may not be required as the CQI for muting can also be approximated by the eNB. For CS/CB the other resource PMI feedback is the required support. It is concluded that the per resource CSI-RS feedback with possible enhanced interference estimates supports a seamless mixture of all the CoMP categories.
However; a concrete CQI feedback design is not presented in that contribution. Either the UE reports both, aggregated CQI for JT and per resource CQIs for DPS, or the CQI reporting mode is configured by the network.
In another contribution, studies are made on different assumptions for CQI calculations for per point CQI reporting with the assumption that from N per point CQIs the eNB derives the CoMP CQI.
Still further, there is a proposal to have per point CQI/PMI feedback with possible combiner feedback to support both, DPS and JT CoMP. The muting option in the CQI calculations remained unconsidered.
An analysis on the performance of JT CoMP with different feedback assumptions was presented in another contribution. The conclusions from that simulations is that non-coherent JT with aggregated CQI can achieve similar performance as coherent JT (with aggregated CQI). Thus, in that scenario, the per point CQIs and the aggregated CQI may not have the same value for JT CoMP performance.
Further contributions are related to CoMP CQI. Several thereof did not discuss the CQI derivation details. Others presented equations on how the eNB can derive the CQI for joint transmission from per point CQIs with a given assumption on interference. The assumptions on how the UE would estimate the interference level for the per-point CQIs was not considered.
Still other contributions discuss measurement objectives as follows “Observation 1: for each CoMP scheme, considering the coordination method, the interference should: Include signals from all the points/cells outside of the transmission set or coordination set; consider the actual resource elements causing interference to the PDSCH; not include signals from the point/cell the UE assumes as transmitting the PDSCH; not include signals from the point/cell with blanking on some or all resource elements”; but no concrete scheme for measurement is shown.
For example in the case of CS/CB, coordination between points reduces the level of interference, though the level of interference reduction is typically not known by the UE.
For describing the problem this invention relates to, the following shall be considered.
The terminal device UE is able to measure the interference and evaluate the channel quality indicators (CQIs) for different CoMP schemes (hence under respective different CoMP hypothesis) and to subsequently pick (or determine for selection and reporting towards the network device eBB) the best CoMP scheme for the UE. (“Best” in the sense that the selected CoMP scheme optimizes the UE's communication “experience” e.g. in terms of highest data rate/throughput, lowest interference or the like).
However, from UE perspective, certain CoMP schemes, e.g. joint transmission, might always be better than the other available schemes. On the other hand, from the perspective of the whole system (represented by the eNB for example) it might be more beneficial to utilize e.g. dynamic point selection or CS/CB (rather than joint transmission (JT) preferred by the UE).
Hence, the system performance might not be fully optimized. Several options for CQI computation and reporting are possible that may avoid such situation:
1) Use of Multiple CQI Values in Reporting:
One would report CQIJT and (one or more) CQInon-JT, covering multiple CQI hypothesis. Hence, in this case, the UE would report CQIs for multiple schemes, but let the eNB decide which to use. CQnon-JT may be computed also as a delta CQI compared to CQIJT. However, from feedback perspective, multiple CQIs (or differences delta between the multitude of CQI values) need to be transmitted. This implies an increased overhead on the feedback (uplink control) channels.
2) Use of a RRC Configured CQI Type
In this case, the eNB would indicate the specific (single) hypothesis for which the CQI needs to be computed. As to computation complexity, the complexity involved is the one resulting for only one hypothesis CQI calculation, and also from the feedback perspective, only one CQI would have to be transmitted. However, flexibility to utilize different schemes and to use those more dynamically based on current interference situations the UE sees, is then lost.
Thus, existing and/or discussed systems for CoMP still leave it unaddressed to control the selection of the CoMP transmission scheme applied at a given time (for transmission from an eNB towards a UE).
Thus, there is still a need to further improve such systems.